There are times when the multiplication algorithm gives you some shortcut multiplications if you just inspect what you are doing. Have your students perform various multiplications with the numbers 21 31 41 51
.
They will soon stumble on a neat little multiplication shortcut.
To multiply by 21: Double the number, then multiply by 10 and
add the original number.
For example: To multiply 37 by 21,
Double 37 yields 74,multiply by 10 to get 740
and then add the original number 37 to get 777.
To multiply by 31: Triple the number, then multiply by 10 and add
the original number.
For example: To multiply 43 by 31,
Triple 43 yields 129,multiply by 10 to get 1,290
and then add the original number 43 to get 1,333.
To multiply by 41: Quadruple the number, then multiply by 10 and
add the original number.
For example: To multiply 47 by 41,
Quadruple 47 yields 188,multiply by 10 to get 1,880
and then add the original number 47 to get 1,927.
By now your students should be able to recognize the pattern. Have them extend the rule further to other numbers.
Taken From :Math Wonders to inspire teacher and student
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